# std::basic_string<CharT,Traits,Allocator>::shrink_to_fit

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< cpp‎ | string‎ | basic string

void shrink_to_fit(); |  | (constexpr since C++20)

Requests the removal of unused capacity.

It is a non-binding request to reduce capacity() to size(). It depends on the implementation if the request is fulfilled.

If (and only if) reallocation takes place, all pointers, references, and iterators are invalidated.

### Complexity

Linear in the size of the string.

### Notes

In libstdc++, `shrink_to_fit()` is not available in C++98 mode.

### Example

Run this code

    #include <iostream>
    #include <string>

    int main()
    {
        std::string s;
        std::cout << "Size of std::string is " << sizeof s << " bytes\n"
            << "Default-constructed capacity is " << s.capacity()
            << " and size is " << s.size() << '\n';

        for (int i = 0; i < 42; i++)
            s.append(" 42 ");
        std::cout << "Capacity after 42 appends is " << s.capacity()
            << " and size is " << s.size() << '\n';

        s.clear();
        std::cout << "Capacity after clear() is " << s.capacity()
            << " and size is " << s.size() << '\n';

        s.shrink_to_fit();
        std::cout << "Capacity after shrink_to_fit() is " << s.capacity()
            << " and size is " << s.size() << '\n';
    }

Possible output:

    GCC output:
    Size of std::string is 32 bytes
    Default-constructed capacity is 15 and size 0
    Capacity after 42 appends is 240 and size 168
    Capacity after clear() is 240 and size 0
    Capacity after shrink_to_fit() is 15 and size 0

    clang output (with -stdlib=libc++):
    Size of std::string is 24 bytes
    Default-constructed capacity is 22 and size is 0
    Capacity after 42 appends is 191 and size is 168
    Capacity after clear() is 191 and size is 0
    Capacity after shrink_to_fit() is 22 and size is 0

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 755 | C++98 | `std::string` lacked explicit shrink-to-fit operations | provided
LWG 2223 | C++98 | 1. references, pointers, and iterators were not invalidated 2. there was no complexity requirement | 1. they may be invalidated 2. required to be linear

### See also

sizelength | returns the number of characters (public member function)
capacity | returns the number of characters that can be held in currently allocated storage (public member function)
resize | changes the number of characters stored (public member function)

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